1. Field of the Invention
The present invention relates to an optical information recording medium and a manufacturing method thereof. More particularly, the present invention is concerned with an optical information recording medium having a protective layer, and an optical information recording medium manufacturing method based on a sputtering method.
2. Description of the Related Art
In recent years, high-definition video data has come to be recorded in optical information recording media. Accordingly, there is an increasing demand for an optical information recording medium offering a higher information recording density. A write-once type optical information recording medium that employs laser light of a short wavelength ranging from about 360 nm to about 450 nm (such as around 405 nm), for example, a Blu-ray disc has been proposed. The optical information recording medium has an organic dye compound such as an azo dye or a cyanine dye employed in an optical recording layer thereof. By absorbing laser light, the organic dye compound is decomposed or degenerated. A change in the optical characteristic of the organic dye compound occurring at the recording/playback wavelength of the laser light is measured as a degree of modulation, whereby recording or playback can be achieved.
The foregoing optical information recording medium has an optical reflective layer and an optical recording layer successively formed or superposed on a resin substrate which has a guide groove (which may be called a pre-groove, but it is abbreviated as “groove” hereinafter) formed in the light incidence side surface thereof. An optical transparency layer made of an optical transparency resin is formed on the optical recording layer. Thus, the optical information recording medium has the same diameter and thickness as a CD-R or a DVD±R does. For protection of the optical recording layer, a protective layer made of an optically transmissive inorganic material is interposed between the optical recording layer and optical transparency layer.
As for the Blu-ray disc having an organic dye coated as the optical recording layer, a fabricating procedure is such that the surface of a resin substrate in which grooves are formed is coated with an optical reflective film in a vacuum according to the sputtering method, is tentatively taken out to the air, is coated with an organic dye according to the spin coat method, is coated with a protective film in a vacuum again according to the sputtering method, and is finally bonded to an optically transmissive sheet. When the optical recording layer is made of an inorganic material, a method of carrying the optical reflective layer, optical recording layer, and protective layer in a vacuum and continuously sputtering them is adopted so that the layers can be accumulated without being taken out to the air.
In a sputtering device to be employed in coating the optical reflective layer and others, a substrate holder 2 for an optical information recording medium and a target presser 6 are, as shown in FIG. 5, employed so that a substrate 1 of an information recording medium that is a workpiece will be opposed to a target 5 which is gold, aluminum, or the like. The substrate holder 2 and target presser 6 are made of a nonconductive resin material for fear when the high-frequency sputtering method is used for coating, the substrate to be sputtered or the target may conduct.
When the substrate 1 of the optical information recording medium is set in place, an inner mask 3 and an outer mask 4 are placed on the center of the substrate 1 and the perimeter thereof in order to mask the coating side of the substrate 1. As a mask material, a metallic material or an alloy material is generally adopted.
FIG. 6 shows another example of the substrate holder, wherein the similar inner mask 3 and outer mask 4 are employed.
Forming the protective layer after the optical reflective layer is coated over the substrate according to the sputtering method is not limited to the aforesaid Blu-ray disc but is performed on any other type of optical information recording medium.
For example, a patent document 1 reads that an optical reflective layer is formed on a playback-only disc, which has the optical reflective layer and a protective layer coated over a disc-shaped substrate made of a synthetic resin and having ruggedness information inscribed therein, according to the sputtering method, the inner marginal part of the substrate and the outer marginal part thereof are shielded with a cover.
A patent document 2 says that a reflective layer is deposited on a recording layer according to the sputtering method or the like in order to produce an optical information recording medium which has at least a recording dye layer made of a dye material, the reflective layer made of a metal, and a protective layer formed successively on a transparent substrate.
Further, a patent document 3 reads that for production of an optical information recording medium of two-layer type having a recording layer that contains an organic dye, an optical reflective layer and a protective layer containing elemental sulfur are not brought into contact with each other. The patent document 3 says that the protective layer is formed according to the sputtering method.
Incidentally, the patent document 1 refers to JP-A-4-14634, the patent document 2 refers to JP-A-11-134714, and the patent document 3 refers to JP-A-2005-267670.
In the optical information recording media described in the patent documents 1 and 2, the sputtering method is adopted only at the step of forming the optical reflective layer. Since the protective layer is formed by spinning an ultraviolet stiffening resin so as to apply it over the optical reflective layer, no problem occurs. However, for manufacture of the Blu-ray disc, after the optical reflective layer and an optical recording layer are coated, when a protective layer is coated by performing sputtering, if the inner mask and outer mask come into contact with the already formed metallic optical reflective layer, the optical reflective layer becomes conducting. Abnormal electrical discharge may take place. In this case, the outer circumferential end surface of the optical reflective layer may be jagged. Otherwise, a lightning-like electrical discharge mark that runs from the outer circumferential end of the optical reflective layer to the inner circumferential side thereof has occurred, or any other unprecedented problem has come to light.
In the optical information recording medium described in the patent document 3, various kinds of protective layers are exemplified. However, a concrete way of masking is not described. No description is made of the foregoing problems.